Search engine and method with improved relevancy, scope, and timeliness

ABSTRACT

A search engine and a method achieve timeliness of documents returned in a search result by a relevancy feedback mechanism driven by the frequency in which a URL is returned in recent searches. The relevancy feedback mechanism includes one or more random processes which determine whether or not a cached or indexed web page associated with a URL in the search result should be refreshed. In addition, the random processes also determine whether or not hyperlinks in the cached or indexed web page should be followed to access related web pages. Accesses of web pages resulting from the operations of the random processes are used to update any document index maintained by the search engine. Relevancy scoring functions implemented in look-up tables are also disclosed. A more accurate relevancy scoring function is achieved using a lexicon based on anchortexts of extracted hyperlinks of web documents.

CROSS-REFERENCE TO COPENDING PATENT APPLICATIONS

This invention relates to and claims priority to U.S. provisional patent application Ser. No. 60/464,744, entitled “Search Engine with Improved Relevancy, Scope, and Timeliness,” filed on Apr. 24, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to search engine technology. In particular, the present invention relates to search engines and methods for quick retrieval of relevant and timely documents from a wide area network, such as the World Wide Web.

2. Discussion of the Related Art

The search engine is an important enabling application of the internet which allows the user to quickly identify and retrieve information (“web pages”) from the World Wide Web (WWW). In fact, the search engine has caused a profound consumer behavioral change: the user now prefers typing his data retrieval criteria into a “search box” to “browsing” or traversing painstakingly and manually cataloged hierarchical directories. Today, more than a hundred million searches are performed every day on the several billion web pages of the WWW. Yet, existing methods remain unsatisfactory in addressing the most basic search problems.

Three desired qualities are fundamental to a search: the relevancy of the search results returned, the extent of the coverage (“scope”) over the WWW, and the age (“timeliness”) of the information retrieved. As to relevancy, as the index size grows current search engines should aim to achieve ever greater refinement and accuracy on the web pages they find and rank, so that the first few web pages returned to a user would correspond precisely to the information the user is seeking. With respect to scope, even the largest search engines index only a fraction of the WWW at the present time. Nevertheless, most of the web pages that are indexed are never returned as search results to actual queries. Thus, search engines should improve the scope of their indexing, especially automatic indexing, so that a greater portion of the useful content that exists on the WWW can be made available and more efficiently accessed. Also, the largest search engines today are unable to refresh their search indexes quickly enough to return only current information from the WWW. Today, these search engines often return many web pages which content are significantly changed from when they were indexed; at worst, some indexed web pages simply no longer exist (i.e., “dead links”).

To improve relevancy, some search engines take a “tiered” approach. Under a tiered approach, a search engine gives greater weight in its indexing to one or more small subsets of the WWW, which are often handcrafted, hierarchical directories that it considers to be of high quality. However, because the web pages in the subsets are manually selected, these web pages often lag in time relative to the rest of the index.

To improve scope, niche “meta-search engines” try to provide an equivalent of a larger search index by combining results from multiple search engines. However, by combining the results of many search engines, these niche meta-search engines erase from the results the effects of the included intelligence or careful tuning of the algorithms in each individual, proprietary search engine. The resulting web pages retrieved are also often ranked in an ad-hoc fashion, resulting in a substantial loss of relevancy.

To improve timeliness, current search engines often identify web pages which content change frequently, and accordingly re-index these web pages more frequently than other web pages. Another approach evaluates a web page's historical change frequency and adaptively accesses the web page at a rate commensurate with the recent change frequency. However, these approaches can manage an index over only a relatively small subset of the WWW, and even then only with limited efficiency. In fact, many changes to a web page (for example, a dynamic time-stamp) may not significantly impact the search results to actual queries. Consequently, much of the WWW “crawling” (i.e., content discovery, also called “spidering”) and updating efforts are believed wasted.

Some solutions to these problems are disclosed in U.S. Pat. Nos. 5,701,256 and 6,070,158 relating, respectively, to proteomic sequences search engine and to phrase-based WWW search engine and meta- or distributed search engines.

U.S. Pat. No. 6,070,158 by William Chang provides an example of the construction of a large-scale search engine.

SUMMARY

The present invention provides web-crawling methods that differ from the prior art in fundamental ways. The methods of the present invention index, update, and rank web pages to achieve relevancy, scope, timeliness and efficiency simultaneously. In one embodiment, a relevance ranking method based on a statistical measure of “confidence of relevance” uses term lexicon and training data extracted from hyperlinks in the WWW.

A search engine in which the actual search results (the “hits”) are cached, including for example meta- or tiered search engines, is used to drive both content discovery and updating in a probabilistic manner. In a search engine of the present invention, the more often a hit is returned, the more often the web page is refreshed and the hyperlinks within its content are explored.

The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a method for constructing a lexicon, in accordance with the present invention.

FIG. 2 illustrates an adaptive relevancy feedback mechanism which ensures both timeliness and improved scope in search results, in accordance with one embodiment of the present invention.

FIG. 3 illustrates an exemplary process for building a relevancy scoring matrix, in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention provides a method for efficient retrieval of data from the WWW based on constructing a lexicon. To construct a lexicon according to one embodiment, which is illustrated in FIG. 1, one starts with an initial collection of web pages (the “corpus”) 101 on one or many subjects (e.g., the entirety or a portion of the Open Directory Project, or a collection of bookmarked URLs¹). The corpus is parsed at step 102 to extract hyperlinks on these web pages and their associated “anchortexts.” After processing to eliminate long or common phrases that should not be indexed, anchortexts are sorted, tallied and “clustered” to create lexicon 103 of “terms”. Each term is deemed to represent a unique concept (such as, for example, a proper name of a person or a product). The terms can be words, phrases or collections of words or phrases. To this lexicon, additional terms can be extracted from the corpus for inclusion into the lexicon or introduced from elsewhere. In this context, clustering refers to the grouping of different anchortexts that refer to a common hyperlinked URL. When two anchortexts each occurring a sufficient number of times with the same URL, the anchortexts are deemed “synonyms.” In addition, lexical, morphological, ¹The acronym URL stands for “universal resource locator,” which is typically a string used in specifying an object on the internet together with a method of access. The familiar string http://www.yahoo.com, for example, is a URL specifying the hypertext document which is the home page of the domain yahoo.com, to be accessed using the “http” protocol.

or syntactic analysis may be used to further cluster. In one embodiment, clustering is also carried out by “stemming” of verb tenses, noun plurals, variations of spelling and word order. For example, the variants “CDROM”, “CD-ROM”, “CD ROM”, and “cdrom” may be considered different forms of the same term in the lexicon. This lexicon may be further enlarged by including in the corpus one or more next levels of hyperlinked web pages (steps 104 and 105), until the lexicon has become sufficiently rich or stable.

According to another embodiment of the present invention, a method provides a relevancy scoring capability for terms in a lexicon. A “relevancy scoring matrix” R(TF, DF) may be constructed to allow looking up a relevancy score for a document matching a given term, using a document frequency (“DF”) and a term frequency (“TF”). In this context, the DF for a term t is a (quantized) percentage of the web pages of the corpus that contain the term t, and the TF of a term t in a given document refers to the number of occurrences—sometimes weighted and normalized—of that term in the document. The relevancy score may be, for example, the product of TF and DF². FIG. 3 illustrates an exemplary process for building a relevancy scoring matrix, in accordance with the present invention.

In one embodiment, matrix R is constructed by first constructing an adequate lexicon, using a method such as the anchortext-based method described above with respect to FIG. 1 (step 301). As discussed above, one first identifies all anchortexts in a selected group of documents of the corpus (e.g., the most recently accessed million documents) as the seed lexicon. The lexicon is then expanded to include anchortexts in the documents referenced by the hyperlinks of the selected group of documents. These referenced documents are then included in the selected group to form an expanded selected group of documents, and the lexicon can be further expanded by following the hyperlinks of this expanded selected group of documents (the “training set”). Of course, the entire corpus may also be used to build the lexicon. This approach that uses anchortexts to seed and expand the lexicon is deemed more reliable than an approach based on a statistical distribution of the terms in the lexicon among documents not known to be relevant.

The relevancy scoring matrix R is then constructed by: ² Gerard Salton proposes using a scoring function which is the product f(TF)*g(DF), where f and g are some functions of TF and DF, respectively. In practice, this scoring function introduces errors when used to provide a ranking of relevant documents, especially when terms of very different DF are combined in a query. (Intuitively, given a common term and a rare term, their respective TF contributions in the scoring function follow different curves; thus, the function f should not be independent of DF, but should be a function of both TF and DF.)

-   -   (1) at step 302, initializing each entry R(TF, DF) in the         relevancy scoring matrix R to zero, for all TFs and DFs;     -   (2) at step 303, for each term in the lexicon and each document         in the corpus that is a priori deemed relevant (e.g., the term         occurs as anchortext of hyperlinks referencing said document),         tallying (i.e., incrementing by one) the entry of the matrix         R(TF, DF) corresponding to the term frequency (TF) and the         document frequency (DF);     -   (3) at step 304, for each DF, divide each of entry R(i, DF) in         the column of matrix R corresponding to DF by the sum of all the         entries in the column; and     -   (4) at step 305, for each DF, replace each entry R(TF, DF) by         the logarithm of the sum of all entries R(i, DF) in the matrix         R, where i is an index less than or equal to TF.

The entries of matrix R thus constructed can be each used as a measure of statistical relevance. For example, if the user issues a query Q having terms t₁, t₂, . . . t_(p) and if the search returns N documents, a measure of each document's relevancy can be obtained by the sum ${\sum\limits_{s = 1}^{p}{R\left( {n_{s},{DF}_{s}} \right)}},$ where n_(s) is the number of times term t_(s) appears in the document, and DF_(s) is the fraction of the corpus containing term t_(s). Other measures of relevancy may, of course, be possible. For example, in addition to tallying over the entire corpus, as in step (2) above, or a selected fraction of the corpus, the tallying can occur over all or a subset of the terms in the lexicon until the matrix R is either sufficiently un-sparse or can be conditioned by smoothing (i.e., local averaging). Additionally or alternatively, a search engine may adopt a priori that the N highest-TF documents for a given term are always deemed relevant, for some fixed N.

In one embodiment, each element of matrix R(TF,DF), referred to as an “odds ratio”, is the ratio of the tally of relevant hyperlinks to the tally of non-relevant hyperlinks. In this context, each hyperlink is sampled or exhaustively selected from the training set using the following criteria: (a) the document d referenced by the hyperlink is independently judged for relevance to its anchortext term t, (b) term t has term frequency TF within the referenced document d, and (c) term t has document frequency DF within the corpus or the training set. Such an “odds ratio” measures the “confidence of relevance” that the matching of a term and a document with a given (TF,DF) is due to the document's relevance and not due to random chance. This approach is analogous to that used in proteomic sequence alignment, except that hyperlinks and anchortexts are used instead of conservative amino acid mutations in homologous protein domains (called the “PAM model”, and is used in the popular software BLAST) to provide the training data for standardized scoring. The use of hyperlink-derived relevancy training set also allows one to tune on a greater scale any scoring function, either by hand or by computation, to ensure that those documents referenced by hyperlinks with a given term as the anchortext will rank or score high for that term.

In a related embodiment, the documents in a training set is associated with “judgments” that specify whether or not a document d is relevant to a term t. These judgments need not be derived from hyperlink data. In that embodiment, each entry R(TF, DF) of matrix R is the ratio of tallies of relevant to non-relevant judgments relating document d and term t where term t has term frequency TF in document d, and term t has document frequency DF with respect to the corpus. Given observed term from frequency TF of term t in a document, and document frequency DF of term t with respect to the corpus, the entry R(TF,DF) of matrix R estimates the odds that the document is relevant to term t.

One advantage of a scoring function such as the confidence of relevance function described above, is that the scoring function is computed for an individual term, but a score for multiple terms can nevertheless be achieved statistically rigorously and meaningfully by a combination of their individual scores. The “odds” (or confidence) that a given document d is relevant to multiple terms may be regarded as simply the product of the individual odds. In the embodiment described above, by taking the logarithm of the individual score, the product of the individual odds can be represented by a simple arithmetic sum of the individual logarithmic odds.

Note that the terms in the lexicon need not be single words, but include phrases (“maximal terms”) as well. Matching an anchortext by its constituent individual words tends to degrade search engine accuracy. A lexicon that is constructed using maximal term matching provides better performance than a lexicon built from individual words.

When processing a query, the search and scoring are carried out using both the maximal terms and the constituent parts of these maximal terms. The contribution of the constituent parts to the scoring function can be suitably down-weighted to account for “double-counting.” Each term in the query is scored against each document returned. (In practice, scoring can be simplified by including in the calculation only the highest scoring documents for each term). For each term, the search engine looks up DF for the term and the TF in each document where the term appears. The values TF and DF are then used to index into matrix R to obtain a relevance or confidence score for the document with respect to that term. The scores of the document with respect to all individual terms in the query are summed to yield the score with respect to the query for that document. The documents are then ranked according to their scores with respect to the query, and the highest scoring documents are returned as hits.

The odds or probability that a web page is relevant for a given term goes up when it is pointed to by a hyperlink in another reliable or “trustworthy” web page and the hyperlink includes the given term in its anchortext. The improvement in odds can be assigned by judging the relevance of randomly sampled hyperlinks having the given term in their anchortexts. Alternatively, a convenient measure can be obtained from a collection of terms each having roughly an equal or similar DF as the given term (i.e. equally common) that are derived from equally reliable or trustworthy web pages. (In general, confidence odds can be assigned for any criterion by applying the criterion to a random sample and then judging its effectiveness; for example, 9 correct results out of 10 applications of the criterion means an odds ratio of 9:1.) In practice, for large classes of terms, this contribution to the “confidence of relevance” by external hyperlinks can be greater than traditional statistically derived scores based on term frequencies. Especially in the context of the WWW, more accurate searches can be achieved when hyperlinks are taken into account.

Using the “confidence of relevance” scoring described above, a method of the present invention incorporates in its relevance ranking the benefits of both hyperlink and tenn frequency approaches, more rigorously than previous methods in the prior art. The consistent scoring function of the present invention (confidence) can be easily modified to incorporate contribution due to additional intrinsic or extrinsic qualities of the web pages in the corpus, so long as these qualities can be quantified in some manner that adds to or subtract from the confidence of relevance score. Furthermore, down-stream ranking, such as by a distributed, tiered, or meta-search engine, is more predictable and accurate, due to the statistical consistency of confidence score as an odds ratio.

According to one embodiment of the present invention, an adaptive “relevancy feedback” mechanism provides search results that are more timely (i.e., consistent with current content or of current interest). FIG. 2 illustrates such an adaptive relevancy feedback mechanism, in accordance with one embodiment of the present invention. As shown in FIG. 2, when a user issues a query, query engine 201 processes the query using search resources such as indices 202 and returns search results to the user. The URLs and the web pages (in whole or in part; for example, using only the extracted title or summary) pointed to by the URLs in the search results are potentially cached in table 203 of “recently accessed” URLs. Table 203 may be indexed, for example, by URLs. Alternatively, to keep table 203 small, table 203 may be indexed by hash signatures of the URLs. If hash signatures are used, the hash function is selected such that, statistically, only very few key collisions may occur. Table 203 also records for each URL the time of last refresh (i.e., the “age” of the last access to the web page). A replacement process removes aged records from table 203 based on, for example, a determination that the age of each URL record to be removed exceeds a preset time. Alternatively, rather than removing aged records, in some applications or for certain URLs, the URL records determined to have an age exceeding the preset time may be automatically refreshed by accesses to the web pages corresponding to the URLs.

The behaviors of two random processes in probabilistic module 204 are governed by the accesses to records of table 203. First, when a URL in the search results is found in a record of table 203 (i.e., the web page corresponding to the URL is recently crawled or refreshed), the URL will be included in refresh list 206 with a probability f(t, . . . ), where f is a probability function of the first random process that depends on the age t of the URL record in table 203. Other parameters of f(t, . . . ) may include, for example, source parameters (e.g., the identities of the user or the crawler to be used), the type of URL that is to be accessed, index size, and workload. If the web page corresponding to the URL is accessed, the URL will be cached in table 203. Crawler 205 accesses the web pages corresponding to the URLs in list 206. If a URL in the search results is not found in table 203, the URL will be included in refresh list 206 with probability f(infinity, . . . ). In addition, whenever a web page corresponding to a URL cached in table 203 is accessed, the hyperlinks within the web page are extracted. The second random process includes in refresh list 206 each URL among these hyperlinks with a probability h(s, t, . . . ), where h is a probability function of the second random process, s is the number of hyperlinks on the web page from which the hyperlinks are extracted, and t is the age of each cached URL (or infinity, if the URL is not cached). Probability function h(s, t, . . . ) may include other suitable parameters.

When the content of a web page is found to have changed in an access or is found to be no longer in existence, this information is forwarded to search engine 201 or its associated index processor to update the indices in indices 202.

The above relevancy feedback method can be used in conjunction with any conventional crawl and refresh mechanisms, such as brute force, user added URLs, and data-mining from such web resources as news, bulletin boards, and weblogs. Table 203 may be used to coordinate several types of crawlers to minimize overlap. A popular web page (i.e., a web page that appears on many search results) is frequently refreshed due to the first random process. Thus, combined with an effective replacement policy, dead links are rarely found in the query results. Even if the web page returned from the refresh operation is the same as that already cached, the amortized cost of refresh is low when compared to the economic value of the frequent accesses to the web page by users of the search engine. Further, the functions f(t, . . . ) can be tuned or throttled to avoid over-refresh. Similarly, probability function h(s, t, . . . ) of the second random process favors accesses to hyperlinks found on a popular web page. Consequently, the second random process is a cost-effective content discovery mechanism.

According to the methods of the present invention, given an ideal index of all documents, obscure web pages are unlikely to be found in actual query results and thus are allocated less resources. Because resources are more efficiently utilized, query engine 201 can perform indexing and querying over a greater and more useful scope. A method of the present invention is especially well-suited to building a tiered search engine, and can quickly transform a meta-search engine into an effective tiered one, by utilizing a table 203 of cached URLs as the basis of a preferred subset. The “relevance feedback” mechanism described above expands the search scope with additional content referenced by hyperlinks in web pages of initial search results. Accordingly, the present invention simultaneously improves future search relevance, scope, and timeliness cost-effectively.

To determine if a web page has changed from the cached copy, rather than parsing each web page retrieved from the WWW and comparing the newly retrieved web page against the cached copy, a signature of the web page can be computed and cached for the web page. When the signatures of the retrieved web page and the cached page are the same, for certain applications or classes of web pages, it may be unnecessary to re-parse the web page or refresh the hyperlinks within the web page. Additionally, the hyperlinks may be saved to facilitate subsequent repeated access under the second random process.

Additional hints, such as the placement of the URL among the search results, or whether or not the title of the web page includes one or more of the search terms in the query, may be used as parameters to the probability functions f(.) and h(.) for better performance. The knowledge that the user actually clicked on a hit (i.e., accesses the corresponding web page), through mechanisms such as cookies, Dynamic HTML, and redirect (possibly in a random sampling), provides additional information that can be used by the search engine to further enhance the probability functions.

If the search engine indexes only a subset of plausible terms for a given web page, the relevance feedback mechanism of the present invention can direct the indexing of additional useful terms extracted from both queries and anchortext, and improve the overall lexicon.

According to another aspect of the present invention, a user may request the same query be repeated or iterated to take advantage of the relevancy feedback process for improved relevance (i.e., a “Try Harder” button). The search may also be conducted off-line, i.e., one or both the WWW search request and the corresponding search results may possibly be sent through e-mail to allow for additional processing time or iterations (e.g, a query may be specified in the subject line of an e-mail to a search engine address). E-mail may be an especially effective and popular medium for conducting search, since many resources are available to allow an e-mail to be easily sent, received, sorted, saved, and forwarded to others.

The above detailed description is provided to illustrate the specific embodiments of the present invention and is not intended to be limiting. Numerous modifications and variations within the present invention are possible. The present invention is set forth in the following claims. 

1. A method for providing a training set to build a statistical relevancy scoring function to be used in a search engine, comprising: (a) identifying an initial set of hypertext documents as a training set of relevant documents; (b) identifying all hyperlinks included in each hypertext document of the training set identified and their associated anchortexts; (c) including the hypertext documents pointed to by the hyperlinks identified in (b) in the training set; and (d) including the anchortexts associated with the hyperlinks indentified in (b) in a lexicon.
 2. A method as in claim 1, further comprising repeating steps (b)-(d).
 3. A method as in claim 1, further comprising ascertaining using an independent method the relevance of the terms in the lexicon to their associated documents.
 4. A method as in claim 1, wherein the training set is used to tune a scoring function.
 5. A method as in claim 1, wherein the lexicon includes terms consisting more than one word.
 6. A method as in claim 1, further comprising clustering of terms in the lexicon.
 7. A method for providing a relevancy scoring function for scoring documents in a search result, comprising: compiling a lexicon including a plurality of terms that can be used in a search query for a search engine; for each term of the lexicon, identifying from a corpus of documents those in which the term appears, and computing a document frequency based on relative numbers of the identified documents and the documents in the corpus; and creating a look-up table, indexed by the document frequency and a term frequency, for storing a value of the relevancy scoring function, the term frequency being the frequency of occurrence of a term in a given document, the relevancy scoring function being a function of the term frequency and the document frequency.
 8. A method as in claim 7, wherein the relevancy scoring function is the product of a function of the document frequency and a function of the term frequency.
 9. A method as in claim 7, wherein the relevancy scoring function represents the odds that a document is relevant to a term.
 10. A method as in claim 7, wherein the relevancy scoring function is compiled by tallying a number of times that a document is adjudged to be relevant to an included term and the number of times a document is adjudged to be not relevant to the included term.
 11. A method as in claim 10, wherein the documents adjudged are each referenced by a hyperlink in which the included term appears in the anchortext of the hyperlink.
 12. A method as in claim 7, further comprising computing the term frequency for each of the identified documents, and the relevancy scoring function being a function of the term frequencies associated with the identified documents.
 13. A method as in claim 7, wherein the lexicon and the corpus are deemed a set of terms and known relevant documents for each term.
 14. A method as in claim 7, wherein the relevancy scoring function is derived from the ratio of two scoring functions.
 15. A method as in claim 12, wherein when the search result is responsive to a query including more than one term from the lexicon, a document returned in the search result is assigned the sum of all values of the relevancy scoring function associated with all the terms from the lexicon included in the query.
 16. A method as in claim 12, wherein the relevancy scoring function is compiled statistically using the entire corpus.
 17. A method as in claim 12, wherein the relevancy scoring function is compiled statistically using a selected fraction of the corpus.
 18. A method as in claim 7, further comprising smoothing the adjacent entries of the look-up table.
 19. An adaptive feedback method for ensuring timeliness of a collection of web pages, comprising: extracting a URL from a search result; and determining whether or not a web page corresponding to the URL is present in whole or in part in the collection, wherein: when the web page is determined to be present in the collection, downloading and replacing the web page in the collection with a first probability; and when the web page is determined not to be present in the collection, downloading and including the web page in the collection.
 20. A method as in claim 19, wherein the first probability depends on an age of the web page in the collection]
 21. A method as in claim 19, wherein the webpages are collected in a cache.
 22. A method as in claim 19, wherein the web pages are indexed in a document index.
 23. A method as in claim 21, wherein the cache incorporates a replacement policy that favors retaining most recently accessed web pages.
 24. A method as in claim 21, wherein the cache is indexed using a hash signature of a URL.
 25. A method as in claim 19, wherein the first probability depends on one of more of: source parameters, a type of the URL, an index size and a workload of a web crawler.
 26. A method as in claim 19, further comprising extracting hyperlinks from the web page corresponding to the URL and downloading the web pages corresponding to the hyperlinks each with a second probability.
 27. A method as in claim 26, wherein the second probability depends on the number of hyperlinks in the web page
 28. A method as in claim 26, wherein the second probability further depends on an age of the URL.
 29. A method as in claim 22, further comprising updating the document index using information obtained from accessing the web page corresponding to the URL.
 30. An adaptive feedback system for ensuring timeliness of a collection of web pages, comprising: a query processor that extracts a URL from a search result; a cache including a process for determining whether or not a web page corresponding to the URL is present in whole or in part in the collection; a web crawler coupled to the processor wherein: when the web page is determined to be present in the collection, the web crawler downloads and replaces the web page in the collection with a first probability; and when the web page is determined not to be present in the collection, the web crawler downloads and including the web page in the collection.
 31. A system as in claim 30, wherein the first probability depends on an age of the web page in the collection.
 32. A system as in claim 30, wherein the webpages are collected in a cache.
 33. A system as in claim 30, wherein the webpages are indexed in a document index.
 34. A system as in claim 32, wherein the cache incorporates a replacement policy that favors retaining most recently accessed web pages.
 35. A system as in claim 32, wherein the cache is indexed using a hash signature of a URL.
 36. A system as in claim 30, wherein the first probability depends on one of more of: source parameters, a type of the URL, an index size and a workload of a web crawler.
 37. A system as in claim 30, wherein the processor extracts hyperlinks from the web page corresponding to the URL and directs the web crawler to download the web pages corresponding to the hyperlinks each with a second probability.
 38. A system as in claim 37 wherein the second probability depends on the number of hyperlinks in the web page.
 39. A system as in claim 37, wherein the second probability further depends on an age of the URL.
 40. A system as in claim 33, wherein the processor updates the document index using information obtained from accessing the web page corresponding to the URL. 